1. Field of the Invention
The invention relates to a power transmission apparatus for helicopters whose speed change ratio can be continuously varied by a traction transmission mechanism.
2. Description of the Related Art
Recently, the demand for commuter helicopters capable of taking off and landing on a heliport in an urban district has been increased. Reduced noise level is a requirement for such a helicopter, and one of of effective measures to reduce the noise level is to reduce the rotational speed of the main rotor.
Japanese Unexamined Patent Publication JP-A 8-198192 (1996) of the same applicant as that of the present invention, discloses a power transmission apparatus comprising a traction transmission mechanism of continuously variable speed change ratio, coupled to an output shaft of an engine, for making the rotational speed of the power transmission apparatus variable. For the purpose of reducing noise of helicopters, the apparatus is designed to reduce only the rotational speed of the main rotor under the condition that the rotational speed of the tail rotor is maintained constant as well as the number of rotations of the engine.
The traction transmission mechanism includes input and output disks which are oppositely disposed so that their rotation shafts coincide with each other; a transmission roller which transmits the rotational power of the input disk to the output disk; and an actuator which changes tilt angle of the transmission roller. Lubricating oil is interposed between the disks and the roller, so that the power is transmitted by means of the shearing resistance of the lubricating oil. The transmission roller is disposed so as to be tilted by the actuator. By continuously displacing the actuator, the speed change ratio can be continuously varied.
In a conventional power transmission apparatus for helicopters, having no traction transmission mechanism, the number of rotations of the engine is reduced with a constant speed change ratio. Hence, the number of rotations of the main rotor is constant, which causes the problem of noise generation.
On the other hand, simple incorporation of such a traction transmission mechanism into a helicopter causes an increase of control gain during high-speed rotation, which is a cause of a generation of large tilting force. As a consequence the tilt angle is suddenly changed, and therefore a slide occurs between the disks and the rotor. When a slide occurs, power transmission ability is lost and the torque of the main rotor is rapidly lowered.
Among traction transmission mechanisms, for example, a toroidal CVT (Continuously Variable Transmission) has power transmission efficiency of only about 90%. Even when such a CVT is combined with a differential gear transmission, the power transmission efficiency of about 97% is obtained at the highest.